BIO 141: Nervous System Study Guide (Chapters 11–13)

Introduction to the Nervous System

Classification of the Nervous System

The nervous system is divided into two main parts, each with distinct structures and functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes all other nervous tissue outside the CNS, such as nerves and ganglia.

Major Types of Nerves

• Cranial Nerves: 12 pairs (approximately 100 billion neurons), primarily associated with the head and neck.

• Spinal Nerves: 31 pairs (approximately 100 million neurons), connect the CNS to the rest of the body.

Characteristics and Structure of Neurons

Neurons are excitable cells responsible for transmitting signals via action potentials.

• Cell Body (Soma): Site of biosynthetic activity; contains the nucleus and organelles. The cytoskeleton, composed of neurofibrils, maintains cell shape and extends into dendrites and axons.

• Dendrites: Branch-like extensions that receive electrical signals from other neurons and transmit them to the soma, increasing the cell's receptive surface area.

• Axon: A single, long process that carries signals away from the soma. Originates from the axon hillock and may have branches called axon collaterals. The axon ends in axon terminals (synaptic knobs), which communicate with target cells.

Major Parts of an Axon

• Axon Collateral: Branches at right angles from the main axon.

• Myelin Sheath: Insulating layer that increases the speed of impulse conduction.

• Axolemma: Plasma membrane surrounding the axon.

• Axon Hillock: Region where the axon originates from the soma.

• Axon Terminals: Distal endings that transmit signals to other cells.

Myelin Sheath and Its Function

• Formed by oligodendrocytes in the CNS and Schwann cells (neurolemmocytes) in the PNS.

• Increases the speed of nerve impulse conduction by allowing impulses to "jump" between nodes (saltatory conduction).

Myelinated vs. Unmyelinated Axons

• Myelinated Axons: Insulated by a lipid-rich myelin sheath, resulting in rapid conduction (15–150 times faster than unmyelinated axons).

• Unmyelinated Axons: Lack myelin; electrical current leaks out and must be regenerated along the entire axon length, slowing conduction.

• Node of Ranvier: Gaps between myelin sheaths where action potentials are regenerated.

Example: Infants have less myelin in the brain, which is why dietary fat is important for early neural development.

Gray Matter vs. White Matter in the CNS

• White Matter: Composed mainly of myelinated axons.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons.

Axon Diameter and Impulse Speed

• Larger diameter axons conduct impulses faster due to lower resistance to ion flow.

Repair in the Nervous System

• CNS Repair: Limited due to rapid astrocyte division (scar formation), microglial activation (phagocytosis and inflammation), inhibitory factors from oligodendrocytes, and absence of growth factors. Some function may be regained by retraining remaining neurons.

• PNS Repair: Schwann cells (neurolemmocytes) facilitate axon regeneration if the cell body is intact. The distal axon and myelin degenerate, while the proximal end forms growth processes. One process survives and is guided by Schwann cells to the target, reforming the myelin sheath.

The Central Nervous System (CNS)

Functions of the CNS

• Integration and processing of sensory information.

• Initiation and coordination of motor output.

• Higher mental functions (e.g., reasoning, memory, emotion).

Major Parts of the Brain

• Cerebrum: Largest part; responsible for higher brain functions.

• Diencephalon: Includes thalamus, hypothalamus, and epithalamus; involved in sensory relay and homeostasis.

• Brain Stem: Includes midbrain, pons, and medulla oblongata; controls basic life functions.

• Cerebellum: Coordinates movement and balance.

Protection of the CNS

• Bony Protection: Skull (cranium) protects the brain; vertebral column protects the spinal cord.

• Meninges: Three connective tissue membranes (dura mater, arachnoid mater, pia mater) surround the CNS.

• Cerebrospinal Fluid (CSF): Cushions and nourishes the brain and spinal cord.

CSF and Its Importance

• Location: Found in the subarachnoid space, ventricles of the brain, and central canal of the spinal cord.

• Functions: Provides buoyancy, protection, and chemical stability.

• Characteristics: Clear, colorless fluid with specific ionic composition.

Blood Supply and the Blood-Brain Barrier

• Importance of Blood Flow: The brain requires a constant supply of oxygen and glucose; interruption can cause damage.

• Blood-Brain Barrier: Selective barrier formed by endothelial cells and astrocytes, restricting passage of substances from blood to brain tissue.

Surface Features of the Cerebrum

• Gyrus (gyri): Elevated ridges on the brain surface.

• Sulcus (sulci): Shallow grooves between gyri.

• Fissure: Deep grooves separating major brain regions.

Lobes of the Cerebrum

• Frontal, Parietal, Temporal, Occipital, and Insula (deep within the lateral sulcus).

Functional Areas of the Cerebrum

• Sensory Areas: Receive and interpret sensory input.

• Motor Areas: Control voluntary movement.

• Association Areas: Integrate information for complex functions (e.g., reasoning, language).

Association Areas and Their Functions

• Prefrontal cortex: Decision making, personality.

• Wernicke's area: Language comprehension.

• Broca's area: Speech production.

• Other areas: Integrate sensory and motor information.

Major Parts of the Diencephalon

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, endocrine functions, and autonomic nervous system.

• Epithalamus: Contains the pineal gland (melatonin secretion).

Parts of the Brain Stem and Their Functions

• Midbrain: Visual and auditory reflexes.

• Pons: Relays information between cerebrum and cerebellum; regulates breathing.

• Medulla Oblongata: Controls vital functions (heart rate, respiration, blood pressure).

Functions and Structure of the Cerebellum

• Functions: Coordinates voluntary movements, balance, and posture.

• Major Parts: Two hemispheres, vermis (midline), and arbor vitae (white matter).

Functions of the Spinal Cord

• Conducts sensory and motor information between the body and brain.

• Integrates reflexes.

Sensory vs. Motor Tracts in the Spinal Cord

• Sensory (Ascending) Tracts: Carry sensory information to the brain.

• Motor (Descending) Tracts: Transmit motor commands from the brain to effectors.

The Peripheral Nervous System (PNS)

Spinal Nerve Plexuses

• Cervical Plexus: Supplies neck and diaphragm.

• Brachial Plexus: Supplies shoulders and upper limbs.

• Lumbar Plexus: Supplies lower abdomen and anterior thigh.

• Sacral Plexus: Supplies pelvis, posterior thigh, and lower leg.

Reflexes

• Reflex: An automatic, rapid response to a stimulus, often protective in nature.

Cranial Nerves: Order, Names, Types, and Functions

Number	Name	Type	Function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movement, pupil constriction
IV	Trochlear	Motor	Eye movement (superior oblique)
V	Trigeminal	Both	Facial sensation, chewing
VI	Abducens	Motor	Eye movement (lateral rectus)
VII	Facial	Both	Facial expression, taste (anterior 2/3 tongue)
VIII	Vestibulocochlear	Sensory	Hearing, balance
IX	Glossopharyngeal	Both	Taste (posterior 1/3 tongue), swallowing
X	Vagus	Both	Viscera control, taste, swallowing, speech
XI	Accessory	Motor	Head and shoulder movement
XII	Hypoglossal	Motor	Tongue movement

Sensation vs. Perception

• Sensation: The detection of changes in the internal or external environment by sensory receptors.

• Perception: The conscious awareness and interpretation of sensory stimuli by the brain.

Touch and Pressure Receptors

• Meissner's Corpuscles: Detect light touch.

• Pacinian Corpuscles: Detect deep pressure and vibration.

• Merkel Discs: Detect steady pressure and texture.

• Ruffini Endings: Detect skin stretch and sustained pressure.

Referred Pain

• Referred Pain: Pain perceived at a location other than the site of the stimulus, often due to shared neural pathways.

Proprioception and Proprioceptors

• Proprioception: The sense of body position and movement.

• Proprioceptors: Located in muscles, tendons, and joints; provide information about limb position and movement.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard academic sources.